Optimizing light intensity

Insufficient light will produce sparse foliage, spindly branches and poor
flowering. Insufficient light can be a result of inadequate lamp size,
type and lighting duration. Having the foliage too far away from the lamp
or foliage being shaded by other plants, are other common causes.

Lamp
size

The input to a lamp is measured in watts (W). The size of the growing area
will partly determine the required wattage - see Table 4.20 & Fig 4.3.

Plant height

The height of a plant must be considered when determining lamp wattage.
Light intensity diminishes rapidly as distance from the lamp increases (Table
4.20 shows the drop in intensity from 1ft to 3ft). For example,
imagine your plants covered an area 0.9 x 0.9 m. You may be tempted to use
a 400W HPS instead of a 600W HPS. The power costs are 50% less, and it
generates less heat. However, if a significant proportion of the foliage
is 3ft from the lamp, the intensity is only ~5,555 lm/sq.ft for that
foliage. This intensity may be insufficient for the plant species being
grown. By choosing a 600W HPS, the intensity would be roughly double -
assuming the same size gap between the lamp and the foliage.

HINT: It is preferable to keep plants
as short and wide as possible. This can be achieved by removing the
‘growing tip’. Also, erect netting at an appropriate height to restrain
upwards growth and promote sideways growth.

Plant
density/ Shading

Shading becomes an issue when plants are positioned too close to one
another. It is generally more productive to plant fewer plants, rather
than more.

Lamp height

Position the lamp as close as possible to the top of plants without
causing photo-respiration or burning of foliage (see Table 4.20 for “minimum
gap” guidelines). This
best utilizes lamp output. Air cooled lights/shades should be used because
they enable lamps to be positioned much closer to the foliage. These are
particularly beneficial for 1,000W lamps (Fig 4.5).

Lamp shades

These help maximize the amount of light being directed toward the plants
(Fig 4.6). Shades need to be hung so that their height can be easily
adjusted as the plants grow. For safety, ensure mounts are securely
fastened to the ceiling.

Reflective material

The use of reflective material on walls and other large surfaces helps
ensure that light is not wasted through absorption.

Light cycles

Many plant processes optimally require a certain daylight duration. For
example, some require ~18 hours of light per day during the seedling and
vegetative phase. However, flowering is then induced by lowering the
lighting duration to ~12 hours per day.
Be aware that growth can suffer if lighting intervals are irregular.
Employ a timer to help ensure consistency. Note also, vital processes
occur during the night (lights off) period so avoid interrupting it by
turning lights on.